[articles/mixin.dd] Add concatenation, statements, types, expressions

[ntrel]

Change genStruct eponymous template to enum template. Add 5 sections: argument concatenation, declarations, statements, types, expressions. The examples added are all taken from the spec. Add Comparison with C preprocessor heading.

Also tweak spec examples: Split mixin statement spec examples into 2. Change for -> foreach and show mixin code reading a variable declared in current scope. Move mixin variable declaration example to MixinDeclaration from statement.dd.

Update: Add 'Domain Specific Languages' section with simple JS example.

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[ntrel]

@maxhaton Thanks, I changed the wording as suggested.

I also:

• Added a section on 'Domain Specific Languages'.
• Moved the 'Generating Declarations' heading to the top.